This seems misleading to me. My understanding is that Slender Man is a fictonal character who was created outside of video games and appears in other mediums besides having appeared in a video game. It's also my understanding that the two girls attempted to kill the other girl because of the lore about having to commit murder to appease Slender Man which they found on a web site. Trying to link this to video games or even a specific video game is tenuous at best.